Rotary drums are used in a variety of applications, typically in mining and agriculture. Generally, material is provided to one end of the drum while it is rotated, then expelled out through the other end. In some cases, the rotary drum includes screens that act to separate materials by size. For example, excavation contractors may use a screened drum to separate site debris into two fractions: a saleable topsoil for farms, nurseries and site-work; and cleaned rock for aggregates or landscaping work. This allows the contractor to resell waste, instead of incurring the cost of sending it for disposal.
These rotary drums, otherwise referred to as trommels when incorporating perforated walls, generally comprise two or more steel “ride rings” forged or machined onto an outside circumference of the drum, and supported by metal or plastic casters. The ride rings are typically very heavy, adding significant weight to the overall structure. To rotate the drum, the casters are turned, either directly or indirectly, by a motor, and the casters in turn spin the drum based on a frictional force of the ride rings against the casters. A typical prior art design is shown in FIGS. 6A and B.
In mining applications, the trommel turns relatively slowly (<1 RPM) due to the nature of the material being introduced, e.g., high impact, high density material such as rock. The rotational speed of the trommel in these applications is limited due to hysteresis and friction in the mechanical components, which can cause heat buildup and damage to the casters if higher speeds are attempted.
Further, because as each ride ring turns with the drum, there is a significant amount of weight being rotated, and the inertia and dynamics of this rotating mass introduce hazards at higher rotational speeds and drum diameters.
Another issue with the traditional ride ring and caster system occurs when using rotary drum sieves (i.e. trommels). In these systems, the ratio of screening length to total length is an important measure of machine efficiency and footprint within a facility. Traditional designs achieve 60%-80% utilization of the total length, with the number dropping lower as the rotational speed of the trommel increases.
A further problem with rotary drums is that they must often be mounted at a decline in order to get material to move through the drum. This creates mechanical thrust, or axial loading, rather than radial loading. In ring and caster systems, this requires the addition of mechanical components, often thrust rollers, to take the thrust.
The drum itself may weigh up to 40 tons, and have a length of up to 80 feet. Because the entire mass is rotating, supporting this load becomes very difficult. The casters used in prior art designs are limited in the load that they may carry, which limits the distance between casters/ride rings. In other words, more ride rings and respective caster assemblies must be used, spaced closer together, when using a very large diameter drum. Further, only a 60% or less utilization ratio is typically achieved with very large diameter drums, because a large portion of the drum's length is used for carrying rather than screening. Other problems with prior art designs include ride rings that are not consistently round, and dynamic forces may cause the drum to bounce, even at very high weights.
What is therefore needed is a large-diameter rotary drum that is capable of operating at higher rotational speeds with improved utilization ratios and which also has reduced weight, cost, footprint and complexity. Using a large drum diameter solution disclosed below pushes the screen utilization ratio above 90%, reducing cost and footprint, and increasing the amount of separation which can occur in a given space.